1. Field of the Invention
The present invention relates generally to a semiconductor device for use in a semiconductor integrated circuit, and particularly relates to a voltage-regulator device in a semiconductor internal boosting circuit in a semiconductor device.
2. Related Background Art
In a semiconductor integrated circuit, an internal boosting circuit that utilizes a power source voltage to generate a voltage higher than the power source voltage is used often. To obtain a desired constant voltage from the boosted voltage in this semiconductor internal boosting circuit, a voltage regulator device that is called a clamp diode has been used conventionally. The voltage regulator device that is called a clamp diode is also is called a Zener diode, which is a device for obtaining a desired constant voltage utilizing a reverse breakdown phenomenon in a PN junction between a semiconductor substrate or a well and an impurity diffusion layer formed on the semiconductor substrate.
For instance, FIG. 6 is a cross-sectional view illustrating an example of a structure of a part where a clamp diode of a semiconductor device disclosed in the JP11 (1999)-307787 A is formed. The following description will refer to this drawing.
In FIG. 6, an active region 3 is provided on an N-type well 1 of a semiconductor substrate. The active region 3 is surrounded by a device-separating oxide film 2 so as to be insulated. A medium concentration N-type impurity diffusion layer 4 to be used as a channel stopper is provided immediately under the device separating oxide film 2, a high concentration P-type impurity diffusion layer 5 is provided in the active region 3, and a thin oxide film 6 is provided on a surface of the active region 3.
Furthermore, an electrode 7 is provided via the device separating oxide film 2 on the upper portion of active region 3 so that the electrode 7 circularly surrounds the active region 3. The high concentration P-type impurity diffusion layer 5 and the electrode 7 are connected with a aluminum wire 9 for a high concentration P-type impurity diffusion layer and an aluminum wire 10 for an electrode, respectively, via openings of an interlayer insulation film 8.
In the configuration of such a clamp diode, a desired constant voltage is determined according to a reverse breakdown voltage applied to the PN junction between the high concentration P-type impurity diffusion layer 5 and the N-type well 1.
However, in the foregoing configuration, among electron/positive hole pairs produced by the reverse breakdown phenomenon at the PN junction between the high concentration P-type impurity diffusion layer 5 and the N-type well 1, electrons tend to be trapped at an interface between the device separating oxide film 2 and the medium concentration N-type impurity diffusion layer 4 in the vicinity of a border between the active region 3 and the device separating oxide film 2, and in a region under the electrode 7 centering around an interface between the oxide film 6 and the N-type well 1 in the vicinity of a border between the active region 3 and the device separating oxide film 2. This produces a state in which a depletion layer tends to extend from a surface of the active region 3 to an inside of the N-type well 1. Consequently, it is considered that, when a high voltage is applied to the PN junction, the depletion layer is thickened virtually at a PN junction surface, which weakens an electric field directed from the N-type well 1 to the high concentration P-type impurity diffusion layer 5, thereby gradually raising the reverse breakdown voltage in the PN junction.
Furthermore, a total quantity of trapped electrons per se increases as a quantity of electric charges flowing from the N-type well 1 to the high concentration P-type impurity diffusion layer 5 increases, and this results in an increase in the breakdown voltage with time, as shown in FIG. 7.
The clamp diode structure shown in FIG. 6 is a structure capable of suppressing the variation with time to some extent, and conventionally in many devices, even if a clamp withstand voltage changes with time as shown in FIG. 7, it is acceptable. However, for a semiconductor integrated circuit that requires a more accurate constant voltage, it still has been insufficient.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a semiconductor device providing an improved effect of suppressing the variation with time of the reverse breakdown voltage applied to the PN junction, particularly, a voltage-regulator device (clamp diode).
To achieve the foregoing object, a first semiconductor device of the present invention includes an impurity diffusion layer formed on a surface of a certain-conductivity-type semiconductor substrate or well, and a device separating insulation film formed at a distance from the impurity diffusion layer. The impurity diffusion layer has a conductivity opposite to that of the semiconductor substrate or well;. A distance between an end of the impurity diffusion layer and an end of the device separating insulation film is defined to be not less than 1.2 xcexcm.
To achieve the foregoing object, a second semiconductor device of the present invention includes an impurity diffusion layer formed on a surface of a certain-conductivity-type semiconductor substrate or well, a device separating insulation film formed at a distance from the impurity diffusion layer, and an electrode formed thereon via a thin insulation film. The impurity diffusion layer has a conductivity opposite to that of the semiconductor substrate or well. The thin insulation film is thinner than at least the device separating insulation film. The electrode terminates at a midpoint between an end of the impurity diffusion layer and an end of the device separating insulation film.
To achieve the foregoing object, a third semiconductor device of the present invention includes an impurity diffusion layer formed on a surface of a certain-conductivity-type semiconductor substrate or well, a device separating insulation film formed at a distance from the impurity diffusion layer, and an electrode formed thereon with a thin insulation film interposed therebetween. The impurity diffusion layer has a conductivity opposite to that of the semiconductor substrate or well. The thin insulation film is at least thinner than the device separating insulation film. The electrode terminates at a midpoint between an end of the impurity diffusion layer and an end of the device separating insulation film. A distance between the end of the impurity diffusion layer and the end of the device separating insulation film is defined to be not less than 1.2 xcexcm.
In the first through third semiconductor devices, the impurity diffusion layer and the semiconductor substrate or well preferably compose a voltage-regulator device. In the second and third semiconductor devices, the electrode preferably is formed to surround the impurity diffusion layer.
Furthermore, in the second and third semiconductor devices, the thin insulation film preferably is formed with not less than two steps.
According to the foregoing configurations, the variation with time of the reverse breakdown voltage applied to the PN junction formed by an impurity diffusion layer and a semiconductor substrate or a well can be suppressed by defining the distance between an end of the impurity diffusion layer and an end of the device separating insulation film to not less than 1.2 xcexcm, and by providing an electrode that terminates at a midpoint between the impurity diffusion layer and the device separating insulation film. Therefore, it is possible to implement a voltage regulator device that is capable of supplying a more accurate constant voltage.